The present invention relates to a cooled field optical system for uncooled infrared telescopes having a primary mirror with a central hole, the field optical system producing an intermediate image of the exit pupil of the telescope at the location of a cooled diaphragm.
Such field optical systems are used, in particular, on large astronomical instruments for the spectrophotometry of celestial bodies in the infrared spectral region and serve to reduce the thermal background radiation, since, with the selection of a suitable diameter for the cooled diaphragm which defines the field of view of the detector used for the detection, the thermal radiation of the primary mirror element of the telescope will effectively be kept away from the detector at the location of the intermediate image of the exit pupil.
However, it is relatively difficult to install a field optical system with intermediate imaging of the exit pupil into a cooled Dewar flask because, in view of the rather great distance of the exit pupil of the telescope from the Dewar flask which is present in the case of large telescopes, it is necessary to operate with long focal lengths. As a rule, therefore, a folding of the optical ray path is necessary. The deflection of the beam by means of the mirror optical system, used preferably in the infrared, to be sure introduces astigmatism in the imaging of the pupil in such a manner as to result in an incorrect adaptation of the cross-section of the beam to the photosensitive surface of the detector, and this reduces the sensitivity of the latter. A remedy for this is provided by aspherically shaped deflection mirrors, which, however, are rather expensive to manufacture.
The object of the present invention is to provide a cooled field optical system of the aforementioned type which provides a compact construction and assures an imaging of the pupil which is free of astigmatism.